1. Field of the Invention
The present invention relates to a current measuring method and a current measuring apparatus for measuring the current generated in a terminal of a semiconductor device. In particular, the present invention relates to a current measuring method and a current measuring apparatus which can at high speed, accurately measure the power supply current generated in the power supply terminal of the semiconductor device.
2. Description of the Related Art
Recently, through the improvement of CMOS technology, a highly integrated semiconductor device with low power consumption has become apparent. High-speed operation elements such as a microcomputer, a memory, and a DSP, can be integrated into the semiconductor device, for example, a system LSI. In this kind of semiconductor device, the power supply current is usually measured when the CMOS IC is not operating. However, a large surge of power flows into the CMOS IC when the CMOS IC operates. The operation of various kinds of circuits inside the chip of such a system LSI can be very complex at high speed, causing the current to increase during the operation. The fluctuation of the voltage in the power supply terminal then increases, thus increasing the possibility of a malfunction of the device.
Furthermore, in the case of a device, which is operated by battery, the life of the battery decreases if the surge of current during operation is large. It is therefore necessary to measure the current including the surge of current during the operation, for a fixed period of time.
FIG. 1 shows a block diagram of a conventional current measuring method. A power supply (VS) 10, which provides voltage to a device (DUT) 20, must provide the desired voltage very accurately. Therefore, a negative feedback type voltage source is generally used for the power supply 10. However, there is a limit in response speed of current supply of the negative feedback voltage source. When the change in the power supply current IDUT is large and fast, the power supply (VS) 10 cannot respond. Therefore, a bypass capacitor CL, which is usually located near the DUT 20, provides a current ICL1 to the DUT 20.
FIG. 2 shows a diagram explaining the principle of a current measuring method. If the power supply output Vo changes, the negative feedback provided in the power supply 10 functions, and a current IPS is provided from the power supply 10. Therefore, the current discharged from the bypass capacitor CL is charged with the current IPS. If the period from the start of a cycle of consumption of the DUT 20 power supply current to the next cycle of consumption of the power supply current, is long, the IPS becomes IPS (ty)=0 during that period. Therefore, xe2x80x9cthe integrated value of the power supply current=X1xe2x80x9d and xe2x80x9cthe integrated value of the current (IPS) provided from the power supply=X2xe2x80x9d becomes equal in each cycle. Thus, by measuring the current (IPS) provided by the power supply 10, the current consumed by the DUT 20 can be measured for each cycle.
However, if the cycle of the change of the power supply current (IDUT) is short, the IPS cannot become xe2x80x9cIPS (ty)=0xe2x80x9d. The influence of the previous measuring cycle remains in the form of residual current and overlapping consumption cycles, so the current of the device DUT cannot be measured accurately by just measuring the current (IPS) provided from the power supply.
FIG. 3 shows another block diagram of a conventional current measuring method. To solve the problems outlined above, a current-voltage converter L2 is provided between the power supply (VS) 10 and the power supply terminal of the DUT 20, so that the power supply current IDUT can be measured directly. An example of apparatus suitable for this kind of measurement is the Current Transformer (CT-1) (Trademark) of Sony/Tektronix Corporation (Trademark). However, the size of the general current-voltage converter 12 is large, as the current-voltage converter 12 has to be located in a position away from the DUT 20 when testing a wafer form DUT. Therefore, the distance between the current-voltage converter 12 and the bypass capacitor CL is large. The result is, if the change of the power supply current is large, the bypass capacitor CL cannot provide the current instantaneously. Then, a large fluctuation in the voltage Voxe2x80x2 of the power supply terminal occurs, thus causing the malfunction of the DUT 20. Also, to calculate the integrated value of the power supply current, a means of integrating the voltage VM is needed.
Therefore, it is an object of the present invention to provide a current measuring method and a current measuring apparatus which overcome the above issues in the related art. This object is achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
According to the first aspect of the present invention, a current measuring method, which measures a device current flowing through a terminal of a semiconductor device, can be provided. The current measuring method comprises charging the capacitor which is connected between the terminal and an earth potential of the semiconductor device, up to a predetermined voltage; setting the semiconductor device to be in active sate by applying a test pattern to the semiconductor device; measuring a potential of the capacitor at the terminal side after Et predetermined test time has elapsed; and judging whether the device current is within a predetermined allowable range, based on the test time, capacitance of the capacitor, and-the potential.
A current measuring method can be provided which further comprises steps of initializing the capacitor to a predetermined voltage; providing a predetermined current to the capacitor; measuring the change of potential of the capacitor at the terminal side of said semiconductor device after a predetermined time has elapsed; and calculating out the capacity of the capacitor based on the predetermined current, the predetermined time, and the change of potential.
A current measuring method can be provided which further comprises steps of initializing the capacitor to a predetermined voltage; providing a predetermined current to the capacitor; measuring the time taken until the potential of the capacitor at the terminal side of said semiconductor device reaches a predetermined change of potential; and calculating out the capacitance of the capacitor based on the predetermined current, the time, and the change of potential. The predetermined current is a known constant current. The predetermined current providing step may provide a known constant voltage to the capacitor through a known resistance.
A current measuring method can be provided which further comprises steps of calculating out the allowable range of potential after a test time has elapsed based on the largest current allowed into the terminal and the capacitance of the capacitor; and judging that the semiconductor device is defective when the calculated potential is out of the allowable range.
A current measuring method can be provided which further comprises a step of supplementing the terminal with a supplementary current, which supplements the device current when the potential of the terminal is lower than a predetermined value. The supplementary current is generated by a constant-voltage power supply, and supplied to the terminal through a diode.
According to the second aspect of the present invention, a current measuring apparatus, which measures a device current flowing through a terminal of a semiconductor device, can be provided. The current measuring apparatus comprises a capacitor connected between the terminal and an earth potential of the semiconductor device; a driver which charges the capacitor up to a predetermined voltage; a pattern generator which operates the semiconductor device; a comparator which measures the potential of the capacitor at the terminal side after a predetermined test time has elapsed; and a means of calculating out the device current based on the test time, the capacitance of the capacitor, and the potential.
A current measuring apparatus can be provided which further comprises a dummy load circuit providing a predetermined current to the capacitor after setting the capacitor to a predetermined voltage; and a means of calculating out the capacitance of the capacitor. This calculation is based on the potential at the terminal side, measured by the comparator after provision of the predetermined current to the capacitor by the dummy load circuit for a predetermined time, and the predetermined current. The dummy load circuit may provide a known constant current to the capacitor. The dummy load circuit may also provide a known constant voltage to the capacitor through a known resistance.
A current measuring apparatus can be provided which further comprises a means of calculating out the allowable range of the potential after the test time has elapsed based on the largest current allowed into the terminal and the capacitance of the capacitor; and a means of judging that the semiconductor device is defective when the potential is out of the allowable range.
A current measuring apparatus can be provided which further comprises a power supply that provides a supplementary current (which supplement the device current), to the terminal when the potential of the terminal is smaller than a predetermined value. A current measuring apparatus may further comprise a diode which connects the power supply and the terminal.
This summary of the invention does not necessarily describe all necessary features. The invention may also be a sub-combination of these described features.